diff --git a/bpx/schema.py b/bpx/schema.py
index 1e2a8a2..3908f6b 100644
--- a/bpx/schema.py
+++ b/bpx/schema.py
@@ -1,4 +1,4 @@
-from typing import List, Literal, Union, Dict
+from typing import List, Literal, Union, Dict, get_args
 from pydantic import BaseModel, Field, Extra, root_validator
 from bpx import Function, InterpolatedTable
 from warnings import warn
@@ -288,7 +288,7 @@ def __init__(Self, **data):
     @root_validator(pre=True)
     def validate_extra_fields(cls, values):
         for k, v in values.items():
-            if not isinstance(v, (float, Function, InterpolatedTable)):
+            if not isinstance(v, get_args(FloatFunctionTable)):
                 raise TypeError(f"{k} must be of type 'FloatFunctionTable'")
         return values